An automated external defibrillator (AED) is described which spends an increased proportion of a rescue in a CPR mode. This is accomplished by use of a single shock protocol which causes the AED to spend less time in shock analysis and delivery activities as compared with the typical multiple shock protocol. An AED of the present invention preferably is configured such that the rescue protocol can be modified or changed easily without the need to remove the battery or use specialized hardware or software. Preferably the shock waveform of the single shock is a biphasic waveform delivering at least 150 Joules of energy and more preferably at least 200 Joules of energy.